1. Field of the Invention
The present invention relates to color conversion method and color conversion apparatus for performing a color conversion so that a suitable color quality can be obtained in the event that image data, which is obtained by an input device for receiving an image to derive image data, is outputted from an output device for outputting (including not only printing outputs, but also output styles such as a display and a printing machine) an image in accordance with the image data, and a color conversion definition storage medium storing therein a color conversion definition with which a preferred color quality can be obtained.
2. Description of the Related Art
In a system in which an original document is taken into a computer by a reversal film and a digital still camera (DSC) and then the received original document is subjected to a correction of color tone, and data for reflection print output such as print and photograph are generated, there is a need of a technical skill of a man of experience in order to obtain a preferred finishing. This technical skill includes such a matter that the skilled person knows sufficiently the association between digital data and finishing, and in addition the skilled person knows xe2x80x9cwhat color tone an image is to be finished intoxe2x80x9d or xe2x80x9cin what manner an image having a desired impressive expression can be obtainedxe2x80x9d.
According to the prior art, in the make-up color separation scanner, a skilled operator obtains a desired finishing color tone upon recognition of characteristics of the scanner system and the output system (a dot recorder/a printing plate/a printing), and the association between an amount of operation of a control dial of the internal color processing system and movement of data, and in addition change of finishing color. According to the general using way of the make-up color separation scanner, at the time of introduction of the make-up color separation scanner, a person of experience of a seller side of the scanner or a person of experience of buyer side produces preset values (basic conditions) of color processing system control parameters (several tens to one hundred several tens of parameters) so that an optimum color conversion for the usage can be obtained. After the adjustment works for several weeks, several sets of preset values classified for each working content are produced. In the actual color separation work, there are produced data for printing wherein a desired finishing is obtained through applying a fine adjustment for each original document in accordance with the preset values.
Data dealt with in the make-up scanner are used, at the input side, in form of CMY density data wherein RGB values of a read sensor are converted into read density, or equivalent neutral CMY density data. The CMY density data are subjected to a gray range setting (high light, shadow balance), a gray gradation conversion, a color correction, UCR, and a K-print generation, and then density values are converted into dot % to form output data. All setting values of those are concerned with reproduced color, and thus it is impossible to optimize a single parameter. What is concerned with the largest number of parameters is the color correction. By way of typical example, false lightness signal, saturation signal and hue signal are extracted from CMY signals, and there are defined functions of determining correction factors associated with CMYK in accordance with the respective signal intensities. The color correction is divided into a main color correction and a selective color correction in accordance with an effective hue range. The main color correction covers a hue range of 180 degrees on each of necessary colors CMY in such a meaning that CMY three primary colors for forming color are necessary or not, that is, brightness and muddiness of color are independently controlled, and covers a hue range of 180 degrees on each of RGB as unnecessary colors. With respect to the selective color correction, there are prepared functions on 6 hues so as to be effective for a hue range of 120 degrees in such a meaning that the same family colors are controlled. The lightness signal, the hue signal and the saturation signal are formed from the CMY signals entered to the color correction section. Accordingly, an alteration of the gradation conversion at the pre-stage of the color correction section involves a change of color correction characteristics. Further, a quantity of correction is effected mutually between the main color correction and the selective color correction, and also among hue channels in the main color correction, and thus work of producing the condition sets needs skill. However, providing the optimum adjustment for those make it possible that a color range capable of being expressed with a print color material is effectively used, stereoscopic effect and depth effect are enhanced, brightness and clearness are expressed, whereby a preferred color reproduction in suitable brightness and memory color are available. This is a reason why this work is addressed as an xe2x80x9cimage formationxe2x80x9d.
On the other hand, in a system comprising a personal computer, and input and output terminals, it is possible to provide an arbitrary combination of an input device and an output device. However, colors expressed by control color signals (RGB, CMYK) depend on devices. On the other hand, U.S. Pat. No. 4,500,919 discloses a system in which color correction is carried out on a conversion basis in a state that it is displayed on a CRT display, while an image representation of interest is produced, and the image representation of interest is converted into an output signal in such a manner that a signal of the image representation of interest is identical to an xe2x80x9cappearancexe2x80x9d. Further, there is proposed a method of obtaining the same color reproduction with any input and output devices by means of expression (a device profile) with a common color space (CCS) in which input and output display device signals are associated with human visual sensation characteristics (expressed by CIEXYZ and CIEL*a*b*). Thus, this conversion can be supported in an OS level of a personal computer, and it becomes established in form of a color management system. Here, device data is converted into color representation in the common color space (CCS) in accordance with the associated reproduced color, and there is performed a conversion from the color representation in the common color space (CCS) to another device data. With respect to both the conversions, as a scheme of mapping to a color range (gamut) in which the device can be reproduced, there are available various techniques, such as mapping considering that the color tone reproduction is important, and mapping maintaining saturation, as well as mapping exactly representing colors. The common color space is associated with the CIE color system, and thus the coincidence of colors between the different devices is basically supported. Further, the common color space makes it easy to perform an adjustment associated with the human visual sensation such as an enhancement of lightness and an enhancement of saturation.
Further there are proposed a method of absorbing a reproducing characteristic difference due to an observation environment to improve a coincidence of appearance (U.S. Pat. No. 5,754,184), and a method of preserving a result of correction in which an image is corrected on an aesthetic basis in the common color space, in form of a three-dimensional table, and it is applied to another subject (U.S. Pat. No. 5,583,665). According to those methods, colors are controlled on a conversation basis on a monitor to obtain a print color in the expected direction when a desired image is obtained. However, it is difficult to implement the preferred color reproduction in suitable brightness and memory color in accordance with those methods as mentioned above in such a manner that a color range capable of being expressed with a color material obtained by a make-up scanner is effectively used, stereoscopic effect and depth effect are enhanced, brightness and clearness are expressed. What makes it difficult to implement such a preferred color reproduction is mainly such a matter that it is difficult for parameters of LCH to provide a fine control from minimum to maximum of a quantity of color materials necessary for obtaining a preferred print finishing. Further, the fact that the saturation range reproducible in a print is not isotropic with respect to the hue and the lightness is also a main cause of the matter that processing of LCH and RGB is not suitable. For example, it is necessary for processing of enhancing saturation to use both the hue alteration and the lightness alteration together. For this reason, the LCH processing needs a three-dimensional simultaneous operation. However, this is extremely difficult. Further, according to the processing system in the common color space, after the processing, a gamut conversion to the real device data is carried out. At that time, the compression processing is performed at an area particularly near the color area boundary. An adjustment at this area is effective particularly for reproductions of saturation and gradation, but in the even that a color reproducing area compression processing is carried out after the color adjustment on LCH, a result of the adjustment is not exactly reflected on a printed color.
As mentioned above, according to the conventional color conversion system, it is difficult to cope with both the applicability to the combination of various devices and the finishing quality.
In view of the foregoing, it is an object of the present invention to provide color conversion method and color conversion apparatus for performing a color conversion capable of obtaining a color output image excellent in color quality even in the combination of various types of input and output devices, and a color conversion definition storage medium storing therein a color conversion definition capable of being suitably used for obtaining a color output image excellent in color quality even in the combination of various types of input and output devices.
To achieve the above-mentioned object, the present invention provides a color conversion method comprising:
a data receiving step of receiving input color data defined by coordinate values within an input device color space depending on an input device for receiving an image to derive image data; and
a data conversion step of converting the input color data received in said data receiving step into output color data defined by coordinate values within an output device color space depending on an output device for outputting an image based on image data, using
a first color conversion definition defining an association between coordinate values within the input device color space and coordinate values of a common color space independent of the input device and the output device,
a second color conversion definition converting a set of values of input coordinate within partial space of at least part of the common color space into a set of values of output coordinate moved in an opposite direction to a direction of a vector wherein coordinate points of a reference white determined within a white area of the common color space are given as a starting point, and coordinate points of a basic color, in which the set of values of input coordinate exists outside xc2x190xc2x0 in hue angle with respect to hue of a basic color, of basic colors determined in a cyan color area, a magenta color area and a yellow color area of the common color space, respectively, are given as a terminating point, and
a third color conversion definition defining an association between coordinate values within the common color space and coordinate values of the output device color space.
Adoption of the second color conversion definition causes that hue is not always preserved, so that it happens that an input color is converted into an output color which is different in hue from the input color. However, preserving faithfully hue does not always implies improvement of color quality. According to the color conversion method of the present invention, it is possible to obtain output color data for providing a color output image which is higher in color quality as compared with a case where hue is faithfully preserved.
Further, according to the present invention, the second color conversion definition performs a color conversion on the common color space. This feature makes it possible to apply the color conversion to various types of input device and output device.
In the color conversion method according to the present invention as mentioned above, it is preferable that in a case where the common color space is L*a*b* color space, or a case where the common color space is converted into L*a*b* color space,
coordinate points (Lc, ac, bc), (Lm, am, bm) (Ly, ay, by) of a cyan basic color determined in the cyan area, a magenta basic color determined in the magenta area and a yellow basic color determined in the yellow area, of said basic colors, satisfy
40 less than Lc less than 65, xe2x88x9225 less than ac less than xe2x88x9255, xe2x88x9235 less than bc less than xe2x88x9260
30 less than Lm less than 60, 40 less than am less than 90, xe2x88x9220 less than bm less than 20
70 less than Ly less than 90, xe2x88x9220 less than ay less than 20, 50 less than by less than 130
and coordinate points (Lw, aw, bw) of said reference white satisfy
80 less than Lwxe2x89xa6100, |aw| less than 10, |bw| less than 10.
Determination of coordinate points for the respective basic colors and coordinate points for the basic white in the respective color areas makes it possible to greatly improve a color quality as compared with a case where the second color conversion is not adopted.
In the color conversion method according to the present invention as mentioned above, it is acceptable that said second color conversion definition converts a set of values of input coordinate within partial space of at least part of the common color space into a set of values of output coordinate standing still in a direction of a vector wherein coordinate points of the reference white are given as a starting point, and coordinate points of a basic color, in which the set of values of input coordinate exists inside xc2x190xc2x0 in hue angle with respect to hue of a basic color, of said basic colors, are given as a terminating point. Or alternatively it is acceptable that said second color conversion definition converts a set of values of input coordinate within partial space of at least part of the common color space into a set of values of output coordinate moved in a direction identical to a direction of a vector wherein coordinate points of the reference white are given as a starting point, and coordinate points of a basic color, in which the set of values of input coordinate exists inside xc2x190xc2x0 in hue angle with respect to hue of a basic color, of said basic colors, are given as a terminating point.
In the event that a set of values of input coordinate exists inside xc2x190xc2x0 in hue angle with respect to hue of a basic color, it is acceptable that no correction is made, or correction is actively made, whichever is appropriate. In any case, it is possible to obtain output data capable of producing a color output image of high color quality.
In the color conversion method according to the present invention as mentioned above, it is preferable that said data conversion step produces a single color conversion definition in which the first color conversion definition, the second color conversion definition and the third color conversion definition are combined, and converts the input color data into the output color data in accordance with the single color conversion definition.
This feature makes it possible to establish high speed conversion.
In the color conversion method according to the present invention as mentioned above, it is preferable that said data conversion step performs a color conversion using a color conversion definition adjusting a range of brightness of an image in a common color space as well as the above-mentioned first to third color conversion definitions.
To achieve the above-mentioned object, the present invention provides a color conversion apparatus comprising:
a data receiving section for receiving input color data defined by coordinate values within an input device color space depending on an input device for receiving an image to derive image data;
a data conversion section for converting the input color data received by said data receiving section into output color data defined by coordinate values within an output device color space depending on an output device for outputting an image based on image data; and
a data output section for outputting output color data after conversion by said data conversion section,
wherein said data conversion section converts the input color data into the output color data using
a first color conversion definition defining an association between coordinate values within the input device color space and coordinate values of a common color space independent of the input device and the output device,
a second color conversion definition converting a set of values of input coordinate within partial space of at least part of the common color space into a set of values of output coordinate moved in an opposite direction to a direction of a vector wherein coordinate points of a reference white determined within a white area of the common color space are given as a starting point, and coordinate points of a basic color, in which the set of values of input coordinate exists outside xc2x190xc2x0 in hue angle with respect to hue of a basic color, of basic colors determined in a cyan color area, a magenta color area and a yellow color area of the common color space, respectively, are given as a terminating point, and
a third color conversion definition defining an association between coordinate values within the common color space and coordinate values of the output device color space.
Adoption of the second color conversion definition causes that hue is not always preserved, so that it happens that an input color is converted into an output color which is different in hue from the input color. However, preserving faithfully hue does not always implies improvement of color quality. According to the color conversion method of the present invention, it is possible to obtain output color data for providing a color output image which is higher in color quality as compared with a case where hue is faithfully preserved.
Further, according to the present invention, the second color conversion definition performs a color conversion on the common color space. This feature makes it possible to apply the color conversion to various types of input device and output device.
In the color conversion apparatus according to the present invention as mentioned above, it is preferable that in a case where the common color space is L*a*b* color space, or a case where the common color space is converted into L*a*b* color space,
coordinate points (Lc, ac, bc), (Lm, am, bm) (Ly, ay, by) of a cyan basic color determined in the cyan area, a magenta basic color determined in the magenta area and a yellow basic color determined in the yellow area, of said basic colors, satisfy
40 less than Lc less than 65, xe2x88x9225 less than ac less than xe2x88x9255, xe2x88x9235 less than bc less than xe2x88x9260
30 less than Lm less than 60, 40 less than am less than 90, xe2x88x9220 less than bm less than 20
70 less than Ly less than 90, xe2x88x9220 less than ay less than 20, 50 less than by less than 130
and coordinate points (Lw, aw, bw) of said reference white satisfy
80 less than Lwxe2x89xa6100, |aw| less than 10, |bw| less than 10.
Determination of coordinate points for the respective basic colors and coordinate points for the basic white in the respective color areas makes it possible to greatly improve a color quality as compared with a case where the second color conversion is not adopted.
In the color conversion apparatus according to the present invention as mentioned above, it is acceptable that said data conversion section uses, as said second color conversion definition, a color conversion definition which converts a set of values of input coordinate within partial space of at least part of the common color space into a set of values of output coordinate standing still in a direction of a vector wherein coordinate points of the reference white are given as a starting point, and coordinate points of a basic color, in which the set of values of input coordinate exists inside xc2x190xc2x0 in hue angle with respect to hue of a basic color, of said basic colors, are given as a terminating point. Or alternatively it is acceptable that said data conversion section uses, as said second color conversion definition, a color conversion definition which converts a set of values of input coordinate within partial space of at least part of the common color space into a set of values of output coordinate moved in a direction identical to a direction of a vector wherein coordinate points of the reference white are given as a starting point, and coordinate points of a basic color, in which the set of values of input coordinate exists inside xc2x190xc2x0 in hue angle with respect to hue of a basic color, of said basic colors, are given as a terminating point.
In the event that a set of values of input coordinate exists inside xc2x190xc2x0 in hue angle with respect to hue of a basic color, it is acceptable that no correction is made, or correction is actively made, whichever is appropriate. In any case, it is possible to obtain output data capable of producing a color output image of high color quality.
In the color conversion apparatus according to the present invention as mentioned above, it is preferable that said data conversion section produces a single color conversion definition in which the first color conversion definition, the second color conversion definition and the third color conversion definition are combined, and converts the input color data into the output color data in accordance with the single color conversion definition.
This feature makes it possible to establish high speed conversion
In the color conversion apparatus according to the present invention as mentioned above, it is preferable that said color conversion apparatus further comprises a definition storage section for storing:
one or more sorts of said first color conversion definition associated with one or more sorts of input device;
a plurality of sorts of said second color conversion definition;
one or more sorts of said third color conversion definition associated with one or more sorts of output device; and
definition designation information designating one second color conversion definition from among the plurality of sorts of said second color conversion definition in association with said input device and/or said output device.
In this case, it is preferable that said color conversion apparatus further comprises a designating section for designating input device and output device, and
said data conversion section converts the input color data into the output color data using the first, second and third color conversion definitions selected in accordance with the input device and the output device designated by said designating section, and the definition designation information associated with the input device and/or the output device designated by said designating section, from among the first, second and third color conversion definitions stored in said definition storage section.
With respect to the first, second and third color conversion definitions, particularly, the second color conversion definition, a plurality of sorts of color conversion definition are stored beforehand, and in addition definition designation information associated with the input device and/or the output device is stored. This feature makes it possible to designate the second color conversion definition as a default when the input device and the output device are designated. In other words, designation of the input device (the first color conversion definition) and the output device (the second color conversion definition) designates the second color conversion. This feature makes it possible to avoid troublesomeness for designating the second color conversion definition in addition to designation of the input device and the output device.
In the color conversion apparatus according to the present invention as mentioned above, it is preferable that said data conversion section performs a color conversion using a color conversion definition adjusting a range of brightness of an image in a common color space as well as the above-mentioned first to third color conversion definitions.
To achieve the above-mentioned object, the present invention provides a color conversion definition storage medium for storing a color conversion definition for converting a set of values of input coordinate within partial space of at least part of a common color space independent of an input device for receiving an image to derive image data and an output device for outputting an image based on image data into a set of values of output coordinate moved in an opposite direction to a direction of a vector wherein coordinate points of a reference white determined within a white area of the common color space are given as a starting point, and coordinate points of a basic color, in which the set of values of input coordinate exists outside xc2x190xc2x0 in hue angle with respect to hue of a basic color, of basic colors determined in a cyan color area, a magenta color area and a yellow color area of the common color space, respectively, are given as a terminating point.
Conversion of color data using color conversion definition (the second color conversion definition referred to in the color conversion apparatus of the present invention) stored in the color conversion definition storage medium according to the present invention as mentioned above makes it possible to obtain a color output image excellent in color quality. Further, this conversion definition is to perform the color conversion on the common color space, and thus it is suitable for various types of input and output devices.
In the color conversion definition storage medium according to the present invention as mentioned above, it is acceptable that said color conversion definition storage medium stores in addition to said color conversion definition (the second color conversion definition referred to in the color conversion apparatus of the present invention), a color conversion definition (the first color conversion definition referred to in the color conversion apparatus of the present invention) for converting coordinate values within an input device color space depending on an input device for receiving an image to derive image data into coordinate values within the common color space, and a color conversion definition (the third color conversion definition referred to in the color conversion apparatus of the present invention) for converting coordinate values within the common color space into coordinate values within an output device color space depending on an output device for outputting an image based on image data.